media: v4l2-dv-timings: prevent possible overflow in v4l2_detect_gtf()

In v4l2_detect_gtf(), it seems safer to cast the 32-bit image_width
variable to the 64-bit type u64 before multiplying to avoid
a possible overflow. The resulting object code even seems to
look better, at least on x86_64.

Found by Linux Verification Center (linuxtesting.org) with Svace.

[Sergey: rewrote the patch subject/descripition]

Fixes: c9bc9f50753d ("[media] v4l2-dv-timings: fix overflow in gtf timings calculation")
Cc: [e-mail]
Signed-off-by: Karina Yankevich <[e-mail]>
Signed-off-by: Sergey Shtylyov <[e-mail]>
Signed-off-by: Hans Verkuil <[e-mail]>

drivers/media/v4l2-core/v4l2-dv-timings.c

···
 		u64 num;
 		u32 den;
 
-		num = ((image_width * GTF_D_C_PRIME * (u64)hfreq) -
+		num = (((u64)image_width * GTF_D_C_PRIME * hfreq) -
 		      ((u64)image_width * GTF_D_M_PRIME * 1000));
 		den = (hfreq * (100 - GTF_D_C_PRIME) + GTF_D_M_PRIME * 1000) *
 		      (2 * GTF_CELL_GRAN);
···
 		u64 num;
 		u32 den;
 
-		num = ((image_width * GTF_S_C_PRIME * (u64)hfreq) -
+		num = (((u64)image_width * GTF_S_C_PRIME * hfreq) -
 		      ((u64)image_width * GTF_S_M_PRIME * 1000));
 		den = (hfreq * (100 - GTF_S_C_PRIME) + GTF_S_M_PRIME * 1000) *
 		      (2 * GTF_CELL_GRAN);